The disclosure relates to voltage converters, and in particular, to a charge shedding circuit.
Unless otherwise indicated herein, the approaches described in this section are not admitted to be prior art by inclusion in this section.
In switch mode power supplies that use multiple phases, one or more of the phases is often shut down at light load to reduce switching losses and improve efficiency. This idea can also apply to a single phase buck converter with large integrated power metal oxide semiconductor field effect transistors (MOSFETS). At light load, sections of the power MOSFETs can be shut off so that the effective gate charge is smaller. This technique, which is called charge shedding or Q shedding, can boost efficiency by a few percent.
In switch mode power converters, light load efficiency is usually accomplished by reducing bias current and skipping pulses. This technique is optimal for very light loads (e.g., <50 ma) where the converter may skip many cycles before turning on. At light to moderate currents, Q shedding is a better way to boost efficiency because the converter can continue switching at its nominal switching frequency while still enjoying the benefit of reduced switching losses.
FIG. 1 shows this effect for different shed ratios. As parts of the MOSFETs are shed, the peak efficiency changes very little, but the current at which the peak occurs falls. In FIG. 1, the buck converter is operating with 5 amps of ripple at 600 kHz. With ⅓ of the MOSFETs switching, the peak efficiency happens when the load is 1.5 amps. When the entire MOSFETs are switching, the peak moves up to 5 amps. The optimal shed point is defined by the intersection of the two curves at 2.65 amps. With the high ripple in the system, detecting this small load current is challenging (the current ripple might be double the shed threshold). Thus, the current must either be averaged or sampled at a definite time within the period to ensure that the measured current is correct.